Recently, many efforts have been made to develop thin, lightweight steel sheets for construction materials and automobile or transportation vehicle members. Such steel sheets are required to have higher strength, for high durability.
However, an increase in the strength of the steel sheets may result in a decrease in the ductility of the steel sheets. Thus, there is a need for materials to help deal with this inverse relationship.
To satisfy this need, much research has been conducted into improving the strength-ductility relationship of steel sheets and, as a result, phase-transformation steels having retained austenite, in addition to low-temperature microstructures, martensite and bainite, have been developed. Examples of the phase-transformation steels include so-called dual phase (DP) steel, transformation induced plasticity (TRIP) steel, and complex phase (CP) steel. Mechanical characteristics such as tensile strength and elongation of phase-transformation steels vary according to the kinds and fractions of a parent phase and a secondary phase and, particularly, TRIP steel including retained austenite has a relatively high tensile strength-elongation balance (TS×El).
Since CP steel, a kind of phase-transformation steel, has a relatively low elongation compared to other steels, CP steel is processable only through simple processes such as a roll forming process. High-ductility DP steel and TRIP steel are processable through processes such as a cold pressing process.
In addition to the above-described phase-transformation steels, Patent Document 1 discloses twining induced plasticity (TWIP) steel, to which carbon (C) and manganese (Mn) are added in large amounts to obtain an austenitic single phase. TWIP steel has a tensile strength-elongation balance (TS×El) within the range of 50,000 MPa % or greater, that is, satisfactory material characteristics.
Such TWIP steel is required to have Mn in an amount of about 25 wt % or greater if the content of C is 0.4 wt %, and in an amount of 20 wt % or greater if the content of C is 0.6 wt %.
If TWIP steel does not satisfy these element content ranges, austenite inducing twining is not stably formed in a parent phase, but ε-martensite having an HCP structure and α′-martensite having a BCT structure are formed in the parent phase, markedly reducing the workability of the TWIP steel. To prevent this, large amounts of austenite-stabilizing elements may be added to stabilize austenite at room temperature. However, if large amounts of such alloying elements are added to TWIP steel, it may be difficult to perform processes such as a casting process and a rolling process on the TWIP steel because of problems caused by the alloying elements and, economically, the alloying elements may increase the manufacturing costs of the TWIP steel significantly.
Thus, there have been attempts to develop so-called “3rd generation- or eXtra-advanced high strength steel (X-AHSS)” having higher ductility than DP steel and TRIP steel or incurring lower manufacturing costs than TWIP steel, even while having lower ductility than TWIP steel. However, results of the attempts have not yet been successful.
In more detail, Patent Document 2 discloses a method (a quenching and partitioning (Q&P) process) for forming retained austenite and martensite as main microstructures. However, as described in Non-patent Document 1, introducing a method of manufacturing a steel sheet using such a method, if the content of C in a steel sheet is low, at about 0.2%, the yield strength of the steel sheet is very low, at about 400 MPa, and the elongation of a final product is merely similar to that of TRIP steel. In addition, although a method of markedly increasing the yield strength of a steel sheet by increasing the amounts of alloying elements, C and Mn, has been introduced, this method decreases weldability because of excessive amounts of the alloying elements.
Meanwhile, alloying elements such as silicon (Si), manganese (Mn), and aluminum (Al) may be added to steel so as to manufacture a high-strength steel sheet having high ductility. However, a high-strength steel sheet including easily oxidizable Si, Mn, and Al may react with even a small amount of oxygen or vapor existing in an annealing furnace, and thus a single oxide of Si, Mn, or Al, or a complex oxide thereof may be formed on the surface of the high-strength steel sheet. This oxide may decrease the wettability of the high-strength steel sheet with zinc (Zn), and thus the high-strength steel sheet may not be plated with Zn in a local region or in the entire region thereof. That is, plating failure may occur locally or in the entirety of a region, and thus the surface quality of the plated high-strength steel sheet may decrease markedly.
In addition, oxides exiting between a plating layer and a steel sheet may decrease the adhesion between the plating layer and the steel sheet, and thus when the steel sheet is processed through a forming process, the plating layer may be separated from the steel sheet, that is, plating separation may occur.
In particular after annealing, the formation of a single oxide of Si, Mn, or Al, or a complex oxide thereof, increases in proportion to the amounts of oxidizable elements such as Si, Mn, and Al. Thus, these problems of plating failure and separation may occur more seriously in high-strength steel sheets having a strength grade of 780 MPa or greater.
To address these problems, Patent Document 3 discloses a hot-dip galvanizing method including: a process of forming iron (Fe) oxides including a single oxide of Si, Mn, or Al, or a complex oxide thereof in a steel sheet, is formed? to a certain depth by directly oxidizing the steel sheet in an oxidizing atmosphere of a direct flame furnace, while annealing the steel sheet at an air fuel ratio of 0.80 to 0.95; a process of annealing and reducing the steel sheet in a reducing atmosphere to reduce the Fe oxides; and a process of hot-dip galvanizing the steel sheet.
In the above-described method, the steel sheet is heated at a high oxygen partial pressure, to induce oxidation of Fe, and thus an oxide layer is formed to a certain depth in a surface region of the steel sheet. In the oxide layer, oxides of elements that are more oxidizable than Fe are formed, and thus Fe does not diffuse to the surface of the steel sheet. However, easily oxidizable elements, that is, Si, Mn, and/or Al, included in the steel sheet below the oxide layer may be diffused to the surface region of the steel sheet as a heating temperature and time increase. At the interface between the oxide layer and the steel sheet, the diffusion may be blocked by the oxide layer, and Fe oxides may react with Si, Mn, and/or Al, thereby reducing the Fe oxides into Fe and forming a single oxide of Si, Mn, or Al, or a complex oxide thereof. Therefore, after annealing, a reduced Fe layer partially including a single oxide or a complex oxide of Si, Mn, and/or Al may be formed in the uppermost surface region of the steel sheet, and an oxide layer formed by the single oxide or complex oxide of Si, Mn, and/or Al may be located below the reduced Fe layer.
Therefore, if a hot-dip galvanized steel sheet is manufactured by oxidizing and then reducing a steel sheet in an annealing process as described above, due to an oxide layer formed below a reduced Fe layer, that is, formed between the reduced Fe layer and the steel sheet, the adhesion between the reduced Fe layer and the steel sheet may be markedly decreased when the steel sheet is processed through a press working process.